There is an increasing demand for coatings on metallic and non-metallic substrates subject to elevated temperatures for protecting the substrates against oxidation and corrosion. Polyorganosiloxane base coating compositions are known to be more heat resistant than conventional organic polymer coatings such as polyester and polyimide coatings. However, even polyorganosiloxane coatings could not withstand temperatures as high as 400.degree. C. or higher in an air atmosphere for a long time and were likely to peel off under such conditions.
Several heat resistant coating compositions have been proposed. For example, Japanese Patent Application Kokai (JP-A) No. 54768/1987 discloses a composition comprising polytitanocarbosilane, silicone resin, and inorganic filler; JP-A 235370/1987 discloses a composition comprising polycarbosilane, silicone resin, and inorganic filler; JP-A 92969/1990 discloses a blend of an organometallic polymer and silicon dioxide; and Japanese Patent Publication (JP-B) No. 50658/1983 discloses the use of a borosiloxane polymer. However, these coating compositions had poor electrical properties at elevated temperatures. Especially the last-mentioned composition experienced a loss of water resistance and-electrical properties at elevated temperatures. These compositions could not meet all the requirements of coating hardness, heat resistance, electrical insulation, and solvent resistance.
There is a need for a coating composition capable of forming coatings which can meet many requirements including adhesion, heat resistance, coating hardness, and electrical insulation.